1. Field of Invention
This invention is related to a process for the preparation of "squaric acid" (dihydroxycyclobutenedione), the compound having the formula: ##STR1## together with the preparation of its complexes and salts. More particularly, the present invention is related to the preparation of these compounds through the reductive electrolytic cyclotetramerization of carbon monoxide in anhydrous aliphatic nitrile solvent media. The resultant compounds potentially are useful as intermediates in the preparation of dyes, polymers, virucides, and as sequestering agents.
2. Description of the Prior Art
Squaric acid (I) was first reported synthesized in 1959 by S. Cohen, J. R. Latcher and J. D. Park, J. Am. Chem. Soc., Vol. 81, p. 3480, through the hydrolysis of certain halogenated cyclobutene derivatives. Squaric acid displays a particularly interesting chemistry partially due to its dianion (II): ##STR2## which may be considered a tetrameric dianion of carbon monoxide, and which has a completely delocalized electronic structure. Consequently, although "phenolic" in nature, the acid is strong (pK.sub.1 =0.6; pK.sub.2 =3.4).
In U.S. Pat. No. 3,833,489 ('74) a process for preparing squaric acid, its complexes and its salts is described, together with a short summary of the state of the art at that time. The method involves passing an electric current through a solution of carbon monoxide in a solvent media selected from the group consisting of amides of phosphoric acid, amides of carboxylic aliphatic acids having from 1 to 10 carbon atoms, aliphatic ethers, cyclic ethers, liquid polyethers and anhydrous ammonia, at a temperature of from about -30.degree. C. to a temperature up to the boiling point of said solvent and at pressures up to about 420 atm, in order to thereby cause the electrolytic cathodic reductive cyclotetramerization of the carbon monoxide; the reaction being carried out under conditions of substantial separation or non-interference of the anodic reactions and reaction products from the cathodic reactions and reaction products. Although the patentees attempted to claim as their operative group of solvent compounds all non-aqueous solvents that will conduct current with a minimum of resistance, their actual work has disclosed that only certain amides, ethers, and ammonia are operative, and that many other classes of compounds are ineffective. Furthermore, their system is severely hampered by the fact that subsequent separation of the squaric product from the reaction system is quite difficult, and thus commercial usage of this system is flawed. Other articles by the same researchers (Gazetta Chimica Italiana, Vol. 102, pp. 818-821 ('72) and Electrochimica Acta, Vol. 23, pp. 413-417, ('78)) have also investigated the influence that specific parameters such as the particular solvent, electrolyte, electrode material, carbon monoxide pressure and reaction temperature have on the yield of squaric acid. They determined that there is a great deal of unpredictability involved in this process, particularly in the properties of the particular solvent employed. Of particular interest was their finding that solvents such as acetonitrile gave poor results (about 2% current efficiency) thus leading to their conclusion that nitriles are ineffective as solvents for the production of squaric based compounds. An additional troublesome problem, particularly in a large scale commercial operation, is that the separation of the squaric acid products from the resulting residue is extremely complicated and difficult when solvents such as DMF are employed. In addition, it has been discovered that when using the preferred class of solvents claimed by U.S. Pat. No. 3,833,489 to produce squaric acid, surprisingly large fluctuations in product yields can result even in the case of substantially identical back to back experiments.
It is therefore an object of this invention to develop a simple, effective and economical process for the preparation of squaric acid, its metal complexes and its salts, by the electrochemical reductive cyclotetramerization of carbon monoxide in anhydrous aliphatic nitrile solvents producing consistently high product yields and relatively simple product isolation and extraction.
It is another object of this invention to provide a process for the preparation and recovery of squaric compounds which makes subsequent product recovery much easier and reutilization of unconsumed starting materials feasible.